In this proposal we will use Drosophila as a model system to understand how circadian (E24 hr) systems adapt to seasonal changes in environmental conditions, with an emphasis on the relatively uncharacterized role of temperature. A major foundation is based on our long-standing work showing that in D. melanogaster the temperature-dependent splicing of the 3'-terminal intron (called dmpi8) from the critical clock component period (per) is a prominent thermosensor that adjusts the distribution of daily wake-sleep cycles, eliciting seasonably appropriate responses. For example, on warm days splicing of the dmpi8 intron is inefficient leading to decreases in per RNA levels, events that prolong midday siesta and likely minimize the risks associated with desiccation during the hot midday hours. Recent progress indicates that multiple suboptimal splicing signals [i.e., 5' and 3' splice sites (ss)] are the basis for the thermosensitivity in the splicing efficiency of dmpi8, and endow D. melanogaster with the ability to prolong its midday siesta into the mid-to-late afternoon, presumably facilitating its adaptation to temperate climates where warm days are typically associated with extended periods of heat. Indeed, temperature-dependent changes in per 3'-terminal splicing efficiency and adjustments in daily wake-activity profiles are absent in several species of Drosophila that are naturally restricted to Afro- equatorial localities, wherein temperature and daylength undergo little fluctuation throughout the year. Consistent with our hypothesis, these non-thermal responsive species have strong 5' and 3'ss on their per 3'- terminal introns. Low temperatures likely stabilize the interaction of splicing factors with suboptimal splicing signals, providing a basis for thermal calibration. We will undertake a multi-faceted experimental strategy that includes biochemical, molecular, cell-culture and whole animal approaches to understand the cis- and trans- acting factors regulating the splicing efficiency of per 3'- terminal introns and how they modulate wake-sleep profiles in Drosophila. Newly identified natural polymorphisms in per that differentially regulate dmpi8 splicing and might vary geographically will be characterized. We will also determine whether the effects of dmpi8 splicing are preferentially mediated from the per-expressing morning or evening brain pacemaker centers and/or the more recently described arousal/sleep neurons. This analysis should provide further insights into our recent discovery that dmpi8 splicing regulates daytime sleep, suggesting novel non-circadian roles for per in modulating wake-sleep states. By undertaking comparative studies using a wide variety of natural populations and Drosophila species, this proposal offers a unique opportunity to integrate studies on gene expression and neural circuits controlling complex behaviors with ecological and evolutionary implications. On a broader perspective, our work suggests that natural selection operating at the level of splicing signals plays an important role in the thermal adaptation of life forms, raising broad implications for alternative splicing programs and transcriptome regulation, issues we will explore using massive-RNA sequencing technology.